Method and apparatus for recovering precious metal from ore



Feb. 2, 1937. c, AXTELL 2,069,206

METHOD AND APPARATUS FOR RECOVERING PRECIOUS METAL FROM ORE Filed Aug.16, 1933 INVENTOR PIES FbAA/K C AXTELL ATTOR NEY Patented Feb. 2, 1937UNITED STATES PATENT oFFlcE METHOD AND APPARATUS FOR RECOVER- INGPRECIOUS METAL FROM ORE Frank C. Axtell, Pasadena, Calif., assignmofthree-fourths to Harold Whealton, Los Angeles, Calif.

, Application August 16, 1933, Serial No. 685,364

16 Claims. (Cl. 209-179) This invention has to do in a general way withIt is another object of this invention to proa method and apparatus forwinning or recovvide an apparatus of the class described which eringprecious metal, such as gold, silver, platican be successfully andconveniently sealed or num, etc., from finely divided ores or fromfinely locked so that the danger of theft by the operadivided earthy,siliceous, or mineral deposits in tors is eliminated.

which they are contained. In line with these objects, I have developed aThe invention is more particularly related to novel type of plate orother form which is espeimprovements in methods for recovering suchcially adapted for use in processes of the general metals from theirores by means of amalgamacharacter above referred to. This plateconsists tlon, that is, by causing these metals to combine in a generalway of a graphite or carbon plate, 10 with mercury. the surface of whichis amalgamated, or, more At present, the method generally employed forspecifically, is provided with a coating of metal this purpose, consistsin passing the finely diy d r oth rw s c at d w th m r ury. vided oressuspended in water over amalgamat- By the term graphite or carbon, Irefer to the ed copper plates, during which operation the freesubstances sold under these names in commerce 15 metals contained in theore combine with, or and in n ral use in the electrical industries. areamalgamated by, the free mercury or the On form o r phite which is knownto the trade amalgam with which the copper plates are coated, as AchesonGraphite is preferred to the ordiand are thus separated from therelatively great nary car for the P p s ereinafter demass of inactive orinvaluable matter constitutscribed by reason of its superior properties,al- 20 ing the bulk of the ore. though carbon is an equivalent of thisgraphite The method just described possesses inherent for this P p n I lim its use. defects, and its operation is subject to certain Broadly,the plates contemplated by this indifllculties, some of which are:first, the necessivention, may be formed of electrically conductive tyfor frequent and 1abor1ous i g of th material, preferably non-metallic.in character, 25

plates in order to restore or maintain their acw h is chemically inertwith respect to a retivity which is impaired by the chemical actionagent which will attack or dissolve the metallic of certain ingredientsin the ore; second, the base of the coating above referred to. fact thatsome varieties of native gold will not, y h term p ate", as used herein,it is to be in the conventional process outlined above, comunderstoodthat I refer to any desired form or 30 bine or amalgamate with mercury;and, third,-the Shape in which the material y be lded o1 necessity forremoving the gold amalgam from or otherwise constructed. the plates byscraping and brushing, all of which In car y g out y invention, I firstcoat or defects and dlfiiculties result in loss of efficiency cover thesurface of e p ite or carbon p te and increased cost of recovery, as iswell underwith an other deposit of a metal c 35 stood by those familiarwith the art. as p zinc. s ver, eta. in act. practical y It is a primaryobject of this invention to proany of the stable metals will serve,although for duce a novel plate or block which is adapted for obviousreasons I prefer t u e such metals as use in a process of the generalcharacter referred Zinc pp 40 to above, and which is of a nature suchthat the The thickn ss 1' the m li depo it may be 40 necessity fordressing the plates is entirely elimof the order of from one to fivethousandths of mat d, an inch, but it will be understood that any thick-It is aiurther object of this invention to pron s m y be empl y w h willbest meet the duce a novel method and apparatus for use in workingconditions. The metallic surface thus conjun'ction'with the plate of theclass described rm i then subjec d to 8 Pr iminary or 45 whereby thevarieties of native gold referred to primary amalgama y treating thesame with above, which will not combine or amalgamate in a solution ofsalt or compound of mercury, prefthe conventional process first referredto, can be ra y y immersing the plate in an aqueous successfully andcompletely extracted. solution of the double y n of ry and It is a stillfurtherobject of this invention, to potassium or mercury and sodium. Asan alter- 50 produce a plate of the class described whereby native, thisprimary amalgamation may be efthe recovered values can be quickly andcomfeeted by using the metallic rf 1' the Pl pletely removed without theusual scraping and as a cathode in an electrolytic solutioncontainbrushing ordinarily employed in the conventionlug mercury. In anyevent the result is a very a1 methods. perfectly amalgamated metallicsurface which 55 adheres with great tenacity to the carbon or graphiteupon which it is deposited. The plates which have been coated with anamalgamated metallic surface of this character are in a conditionsuitable for use in any apparatus designed for the recovery of preciousmetals by amalgamation, with several inherent advantages over theconventional copper plates, which will be hereinafter more fullydiscussed.

In addition to the plates, however, I have devised a novel method andapparatus in which they are especially adapted for use. In thisconnection, my invention contemplates a method and apparatus whichcombines electrolysis with amalgamation and an apparatus wherein thismethod can be conveniently and efflciently to, which are held in spacedrelation with each other by spacing means of insulating material. Theplates are arranged in staggered relation to each other, so that aliquid suspension of the material delivered into the top of the cellflowsdownwardly and back and forth between the various plates 'until itis discharged from the lower end portion of the case. During thisoperation, the top and bottom plates of the cell are connected across anelectric circuit in amanner such that the metal coated surfaces of theplates form the cathodes and the uncoated surfaces the anodes of aseries cell.

The details in the -method and a preferred form of apparatusfor-practicing the same will be best understood from the followingdescrip tion of the accompanying drawing which is chosen forillustrative purposes only, and in which- Fig. 1 is an enlargedperspective view showing a segment of a preferred fOllIl of platecontemplated by this invention';

Fig. 2 is a perspective view with parts broken away showing a'preferredform of cell contemplated by this invention;

Fig. 3 is a sectional elevation taken through the cell shown in Fig. 2;and

Fig. 4 is'a sectional elevation which may be considered as having beentaken in a plane represented by the line 4-4 in Fig. 3.

More particularly describing the invention as herein illustrated,reference numeral N indicates a plate which isformed of carbon graphiteor other suitable material and is shown as having ribbed or corrugatedsurfaces 12 and I2. The margins and edges of these plates are preferablyground or otherwise finished to the exact dimen-v sions required, andthe side margins of the plates as indicated at l3, are either groundflat or cast flat so as to provide smooth bearing or supportingsurfaces.

In place of being ribbed or corrugated, the surfaces l2 and I2 of theplates may be furnished with depressions or pocketsdesigned to hold 'anexcess of mercury or amalgam. Any desired number of these platesformedin this manner are then coated or covered upon one side only, asindicated at M, with a metal, such for example as zinc. This coating maybe applied in any suitable manner, such as by electrodeposition asexplained above, and the metallic surfaces thus formed are subjected tothe preliminary or primary amalgamation hereinbefore described.

The amalgamated surfaces formed in this manner are then charged with afurther quantity of mercury in the metallic state, any excess thereofbeing retained by the hollows or depressions of the corrugated surfaceswhich thus present an excellent mercurial surface for the amalgamationof free metals. In this connection it is to be understood that while Ihave herein described and illustrated the plates as being corrugated,that any other form of irregularities or depressions may be provided orthe plates may be made substantially fiat.

With regard to the manner of applying the metallic coating, it should bepointed out that the electro-deposited metals appear to combine oramalgamate with mercury with much greater facility than do the samemetals when in the form of castings, rolled sheets, etc., and theyfurther seem to have the eflect of greatly lowering the surface tensionof the mercury.

\ As mentioned above, the plate or plates prepared in this manner may beused with certain advantages in the conventional process of extractingmetals by amalgamation, but I prefer to employ them in a novel type ofapparatus which is exemplified in Figs. 2 to 4 inclusive whereinreference numeral l6 indicates a cell or box composed of insulatingmaterial such as rubber and shown as having an .open top and a removableside closure l1.

In this particular form of the invention, the plates I I are formed withone dimension equal to one dimension of the box, and the other slightlyshorter than the other dimension of the box so that they can besuperimposed with insulating strips l8 and I8 between the bearingsurfaces I3 of the plates. In placing the plates in the box in thismanner, I arrange them so that they are staggered as indicated in Fig.4, and the insulated strips I! are of a nature such that they not onlyspace the plates to provide gaps there-' between, but they efiectivelypack the edges of .the plates so that a liquid passingdownwardly sourceof direct electric current indicated at S. These strips serve assupports for the lower graphite carbon plate HL and the ground orfinished bearing margins l3, which rest directly upon the strips, thusmakeelectrical contact therewith.

In positioning the plates in the cell, the insulating strips are placedon the-ground edges of the bottom plate HL and the next plate ispositioned with its edges-on these strips and with its metallic coatedsurface uppermost, the lower surface thereof being uncoated. Thisprocedure is followed until the desired number of plates have beenplaced in the cell and the top or positive plate is formed of uncoatedgraphite or carbon.

To complete the cell in this form of my invention, I employ a pair ofupright-carbon plates 23 and 23' which rest on the bearing edges l3 ofthe top plate, such plates being connected through a suitable conductor24 with the positive side of the source of the electrical potential S.

For the purpose of sealing the plate assembly within the cell, I mayemploy a hollow cover ,60 metallic strips 22 and 22' which areconnected. -by suitable means with the negative pole of a frame of thetype indicated at 25, such frame having its edges bearing against thetop of the cell and having inwardly extending flanges 26 which engagethe tops of the plates 23 and 23'. This cover frame may be sealed orlocked to the four sides of the cell by means of suitable locks orsimply by placing wires 21 through eyes 28 and 28, the wires beingprovided with pressed metallic seals indicated by reference numeral 29.

In operation, the finely divided ore, suspended in a suitable diluteelectrolyte, such as a solution of sodium chloride or natural groundwater which always ,contains ionized salts, is passed into the top ofthe cell at such rate that the level of the electrolyte in the cellstands constantly above the upper positive plate, the maintenance. ofsuch level being further regulated by adjustment of the gate H at thedischarge opening 20. The electrolyte, in passing downward through thecell, completely fills the spaces or gaps between the adjacent plates,thus establishing and maintaining electric and electrolytic contactbetween all of the plates so that the apparatus functions as a serieselectrolytic cell. During operation, a direct current of electricitypasses through the cell and by its action serves to maintain theamalgamating surfaces of the plates at negative potential so that theyconstitute cathodes of the cell. These surfaces are thereby maintainedin a highly active condition for causing or assisting in theamalgamation of any metallic particles contained in the ore. It has beenfound that a current-density of 10 or 15 amperes per square foot of areaof the negative amlagamated surface of the plates is amply sufficientfor the purpose.

In order to make a clean up, that is, to remove the rich amalgam fromthe plates after this step of the process has been completed, the platesare removed from the cell and in the event they were first coated withan electro-deposit of zinc, they are immersed in a solution ofhydrochloric acid of suitable strength, the zinc thereby being dissolvedand the rich amalgam falling to the bottom of the vessel in which theacid is contained. The stripped plates are removed from the acid,thoroughly washed with .water, and recoated and amalgamated for furtheruse.

The rich amalgam is also removed from the acid,

washed with water, dried, and retorted in the ordinary manner orotherwise treated for the extraction and recovery of the valuable metalswhich it contains.

When the graphite or carbon plates have been coated with a metal otherthan zinc, the reagent used is one which will attack that metal andwhich will not attack the precious metal in the amalgam. For example, ifcopper has been used as the base coat, the plates may be immersed in amore or less dilute solution of nitric acid. The plates may also bestripped by electrolysis, that is, by using them as anodes andsubjecting them to the action of an electric current in a properlyselected electrolyte.

Some of the advantages possessed by the process and apparatuscontemplated by this invention are that an amalgamated plate may be usedwhich is practically chemically pure and that such plate may bemaintained in this condition by the application thereto of the electriccurrent. Furthermore, there is an additional efficiency developedthrough the reducing action of the current on any coated metallicparticles which come in contact with the amalgamated plates. Anothergreat advantage of the process is the ease with which the rich amalgammay be reappended claims.

I claim as my invention:

1. For use in the recovery of a metal from its ore: a carbon plate; anda mercury alloy coating on asurface of said plate.

2. For use in the recovery of a metal from its ore: a carbon platehaving an electro-deposited metal coating on one surface thereof, saidcoating being alloyed with mercury, another surface of said carbon platebeing uncoated.

3. For use in recovering a metal from its ore: a carbon plate having acorrugated surface; and

a metal coating on said surface, said coating com-- prising a chemicallyactive metal and an alloy of mercury.

4. For use in recovering a metal from its ore: a carbon plate having acorrugated surface; a chemically active metal coating on said surface;and an amalgamated surface on said metal coat- 5. For use in therecovery of a metal from its ore: a series cell of the class describedembodying: a box; a plurality of spaced carbon plates in said boxarranged to be connected in an electric circuit so as to have oppositelydisposed cathode and. anode surfaces; and mercury alloy coatings on thecathode surfaces of said plate.

6. For use in the recovery of a metal from its ore, a series cell havinga plurality of plates formed of a nonmetallic electrically conductivematerial, and mercury alloy coatings on the alternate surfaces of saidplates.

7. For use in the recovery of a metal from its ore, a series cell havinga plurality of plates formed of a nonmetallic electrically conductivematerial, coatings on the alternate surfaces of said plates, saidcoatings comprising an electrically conductive material which has anafiinity for the metal in said ore; and means for applying electricpotential across the plates in said cell.

8. A cell for recovering metal from ore embodying: a case; a pluralityof nonmetallic electrically conductive plates mounted in said case andarranged to be connected in an electric circuit so as to have oppositelydisposed cathode and anode surfaces; insulating spacers mounted betweensaid plates and forming gaps therebetween; metallic coatings on thealternate surfaces of said plates, said coatings being composed of metalwhich has an afinity for the metal in said ore; and means for applyingelectric potential across the plates in said cell.

9. A cell for recovering metal from ore of the class describedembodying: an upright case formed of insulating material; a plurality ofsubstantially horizontal electrically conductive plates mounted in saidcase in staggered relation with each other and arranged to be connectedin an electric circuit so as to have oppositely disposed cathode andanode surfaces; insulating means between the edges of said plates forholding same.

in spaced relation with each other; coatings formed of metal having anamnity for the metal in said ore on the alternate surfaces of saidplates; and means for applying electric potential across the plates insaid cell.

10. A cell for extracting metal from ore of the class describedembodying: an upright case; a plurality of substantially horizontalcarbon plates mounted in said case in staggered relation with eachother; insulating means between the edges of said plates for holdingsame in spaced relation with each other; a source of electricalpotential;

- means connecting said plates across said source of potential wherebysaid plates have oppositely disposed cathode and anode surfaces,metallic coatings having an aflinity for the metal in said ore on thecathode surfaces of said plates, said case having an outlet opening inits bottom portion; and an adjustable gate on said outlet opening.

' 11. For use in the recovery of a metal from its ore: a cell havingspaced plates therein and amalgam coatings on the surfaces of saidplates, said plates being composed of non-metallic electricallyconductive material which is chemically inert with respect to a reagentwhich will dissolve said amalgam; and'means for applying electricpotential across the plates in said cell.

12. The method of extracting a metal from its ore which includes:electrolytically depositing saldmetal upon the amalgamated surface of ametal coated carbon plate; and'then immersing the plate in a reagentwhich will react chemically with and dissolve the metal coating on saidplate but which will not attack the material of the plate, the recoveredmetal'or the mercury of the amalgam. I

13. The method of extracting a metal from its ore which includes:electrolytically depositing said metal from said ore onto theamalgamated surface'of a metallic coating formed on a nonmetallic,electrically conductive plate; and then v immersing said plate in areagent which will react chemically with and dissolve the coating metalbut which will not attack the plate or the recovered metal.

14. The method of extracting a precious metal from its ore whichincludes: depositing said metal from the ore onto the amalgamatedsurface of a zinc coating formed on a carbon plate; and then immersingsaid plate in a solution of hydrochloric acid.

-15. The method of extracting a precious metal from its ore whichincludes: forming an amalgam coating on a carbon plate; depositing saidprecious metal from its ore onto said amalgam coating; then strippingthe deposited metal from said plate by immersing said plate in anelectrolytic bath, connecting same as an anode in an electric circuitand subjecting same to the action of an electric current.

16. In a cell for extracting metal from ore of the class described, thecombination of: a pair of carbon plates; insulating means for supportingsaid plates in spaced relation with each other;- means for applyingelectric potential across said plates so that their oppositely disposedsurfaces constitute the cathode and anode respectively of anelectrolytic couple; and a mercury amalgam coating on the cathodesurface on one plate facing the anode surface on the other plate.

FRANK C. AXTELL.

